Either the error was coming with upgrade to 7.10 or it was there before w/o me recognising it.
When the WYSIWYG Filter module is activated and I go to admin/structure/block/add I'll get the following error:
Fatal error: Unsupported operand types in /var/www/vhosts/lokarabia-stage.de/httpdocs/modules/filter/filter.module on line 683
which code is
$filter->settings += $filter_info[$name]['default settings'];
It seems some settings probably have a wrong format, but I'm quite new to Drupal and therefore cannot easily locate the error or wrong setting format.
Tell me if and how I can be of further help to debug it.